Young activists have said they are "willing to sacrifice" their education in order to get action on climate change.
Hundreds of pupils in Wales left school to take part in the global "climate strike" day wanting "an end to fossil fuels and climate justice".
"We're not going to stand for inaction any more," said activist Beth Irving.
"If I had any other option I wouldn't be - as some people have put it - skiving off school," she added.
"Something has got our attention and people have sat up and listened to us because we have broken that status quo and are challenging the system.
"We're willing to sacrifice our own education in order to get people to talk about it and take action and there's something powerful about young people sacrificing their own education - and that's why it has come through.
"I don't think we would have the same effect if we weren't going to school on Friday."
Beth was part of the protest in Cardiff as Welsh activists joined the UK Student Climate Network in one of 11 demonstrations across Wales - including in places including Swansea, Wrexham and Brecon.
#ActOn this #ClimateEmergency this #NatureCrisis unless we demand it. Let your voices be heard...— Woodland Trust Cymru 🏴 🌎 (@CoedCadw) September 20, 2019
“The #ClimateEmergency and a National Forest for #Wales” 💚🌎🌱🌿🌳🌸🐝 Petition: https://t.co/N9qoIj0goC
We’re supporting the #ClimateStrike #YouthStrike4Climate today! #Cardiff pic.twitter.com/BBw0hn1SnP
Pupils will be marked as absent by schools but the leader of one of Wales' biggest councils has urged schools to use "common sense".
"If I was still of school-going age, then I would be on strike today," said Cardiff council leader Huw Thomas.
"Whilst schools in Cardiff have been advised to mark pupils on strike as 'absent', we would hope that schools show common sense and leniency with those pupils who march for the climate - they should wear their absent marks as badges of honour."
The protests in Wales started in the Gwynedd seaside town of Tywyn before spreading to Monmouth.
"We want to show the government and assembly that we aren't going to stand for inaction on climate change any more," Beth told BBC Radio Wales.
"We want to see change happen and happen now. Wales' declaration of a climate emergency is important - but it's not enough.
"What we haven't seen is action to combat that; the Welsh low-carbon plan was put in place before that declaration and hasn't been adapted to show the severity of the crisis."
Wales' future generations commissioner agrees with the students and said the country is "failing to take the action at the scale or pace needed".
"The impact of climate change is already evident in Wales with 23% of our coastline being eroded because of rising sea water as well as the risk of losing one in 14 of our wildlife species," said Sophie Howe.
"Yet it is still our younger generations who are leading the debate and holding governments to account on their lack of action to combat climate change."
Ms Howe said she supported the climate strike and young people's "dedication to keeping the climate emergency firmly on top of the global agenda".
Wales' children's commissioner Sally Holland also showed her support for the protestors and said: "I'm proud of the young people who are once again taking a stand against climate change today."